Primary lithium batteries are divided into high-voltage and low-voltage batteries according to open-circuit voltage. If the open-circuit voltage is greater than or equal to 3.0V, it is high-voltage primary lithium battery, e.g. lithium-thionyl chloride (Li/SOCl2) batteries, lithium-manganese dioxide (Li/MnO2) batteries and lithium-lithium carbon fluoride(Li/CFX) batteries; if the open-circuit voltage is less than 3.0V, it is low-voltage primary lithium battery, e.g. lithium-iron(II) disulfide (Li/FeS2) batteries.
Since Li/FeS2 batteries are low-voltage primary batteries and have an operating voltage platform of 1.5V, they have interchangeability with alkaline manganese (Zn/MnO2) batteries, Ni-MH batteries, carbon batteries or zinc silver batteries having the same size. However, Li/FeS2 batteries have more excellent properties, e.g. Li/FeS2 batteries have higher mass ratio energy, AA-type Li/FeS2 batteries have a mass ratio energy as high as 310 W·h/kg; alkaline manganese (Zn/MnO2) batteries, Ni-MH batteries, carbon batteries or zinc silver batteries having the same size have a mass ratio energy of only 55˜154 W·h/kg. Li/FeS2 batteries have better low-temperature performance than common Zn/MnO2 batteries. Because of aqueous electrolyte, the suitable occasions for Zn/MnO2 are those at a temperature of higher than 0° C.; while Li/FeS2 batteries can still work under conditions of −40° C. Therefore, Li/FeS2 batteries have better market prospects.
Since the negative electrode of Li/FeS2 batteries is lithium or lithium alloys, water will destroy SEI film (solid electrolyte interface film) on the surface of the negative electrode and affect the electrical properties and storage life of batteries, so that water in each part shall be strictly controlled. However, water in the electrolyte is hard to control. For example, the electrolyte solvents DME and 1,3-dioxolane can be dehydrated via molecular sieves to control the water less than 10 ppm. Water in salts (primarily anhydrous lithium iodide) of the electrolyte is hard to remove.
At present, the process for preparing anhydrous lithium iodide (LiI) generally comprises two steps: first synthesizing lithium iodide (LiI.xH2O, wherein x is 0.2-3) containing crystal water; second, removing crystal water in LiI.xH2O. The invention patent having publication No. CN103137981A discloses preparing lithium iodide solids containing crystal water by using elemental iodine, iron powder and lithium hydroxide, then dissolving lithium iodide solids containing crystal water in an organic solvent, then electrochemically electrolyzing under actions of catalytic reduction electrode and lithium electrode, filtering after electrolysis, removing the organic solvent from filtrate to obtain anhydrous lithium iodide. The invention patent having publication No. CN101565192A discloses dehydrating lithium iodide solution to lithium iodide powder containing 0.5-1 crystal water, then vacuum heating and dehydrating to obtain anhydrous lithium iodide.
At high temperature, LiI.xH2O will be easily hydrolyzed and oxidized to produce impurities such as lithium hydroxide, elemental iodine, hydroiodic acid. Therefore, anhydrous lithium iodide products prepared by two steps (preparing LiI.xH2O in a first step and removing crystal water in a second step) have shortcomings of low purity, low yield, and trace water in prepared anhydrous LiI. The shortcomings of low purity and trace water in prepared anhydrous LiI will directly affect the electrical properties of Li/FeS2 batteries, even make the prepared batteries scrapped. Low yield of anhydrous LiI will render a higher cost of formulated electrolyte.
Currently, Li/FeS2 battery electrolyte is formulated by two steps: first, homogeneously mixing anhydrous solvents in certain ratio; second, adding a certain amount of anhydrous lithium iodide prepared or purchased into the solvents and mixing the same homogeneously, to obtain Li/FeS2 battery electrolyte. However, such formulation process has the following problems: first, the anhydrous lithium iodide prepared or purchased has a high cost; second, two steps of such process readily result in introduction of water or new impurities during the formulation.